1. Field of the Invention
The present invention relates generally to optical signal modulation.
2. Description of the Related Art
Passing two (or more) electromagnetic input signals through a nonlinear process is known to generate intermodulation products whose frequencies are the sums and differences of integral multiples of the frequencies of the input signals. For example, if the input signals have angular frequencies of .omega..sub.1 and .omega..sub.2, the intermodulation products will have frequencies of n.omega..sub.1 .+-. m.omega..sub.2. In particular, sum and difference signals are generated which have frequencies of .omega..sub.1 +.omega..sub.2 and .omega..sub.1 -.omega..sub.2 respectively. Other exemplary intermodulation products are third-order products having frequencies such as 2.omega..sub.1 -.omega..sub.2 and 2.omega..sub.2 -.omega..sub.1. In addition, a nonlinear process typically generates input signal harmonics having frequencies such as 2.omega..sub.1 and 2.omega..sub.2.
The nonlinearities of semiconductor diodes have been extensively employed to construct radio-frequency mixer structures. In an exemplary mixer use, an intermediate frequency (IF) signal is mixed with a local oscillator (LO) signal to generate (typically with filtering to remove other products) a radio-frequency (RF) signal, i.e., the IF signal is upconverted by the LO signal to form an RF signal. Conversely, an RF signal can be mixed with an LO signal to generate (again with filtering to remove other products) an IF signal, i.e., the RF signal is downconverted by the LO signal to form an IF signal.
In many applications, it is desirable to transport the upconverted or downconverted signal to a remote site, i.e., a site that is removed from that of the modulation process. For example, space is typically limited in the region of a phased-array antenna so that upconverted and downconverted signals are often generated at the antenna and then transported between the antenna and signal processing circuits which are spaced away from the antenna.
Because radio-frequency transmission lines (e.g., coaxial cables) are relatively lossy (especially at microwave and millimeter-wave frequencies), optoelectronic systems have been developed to perform this transmission between sites with optical carrier signals in optical fibers. Optical fiber loss is typically much reduced from that of radio-frequency transmission lines.
An example of such an optoelectronic system is "antenna remoting" in which radio-frequency signals (often generated by mixing operations) are modulated onto optical carrier signals with electro-optic modulators (e.g., directional coupler and Mach-Zehnder modulators formed in lithium niobate substrates). The modulated optical signal is conducted with an optical fiber between a signal-processing site and a remotely located antenna which is part of a communication system (e.g., an antenna site on a mountain peak with links to other communication sites such as communication satellites). The radio-frequency signals can be recovered at either site by detecting the optical carrier with a photodetector.
Optoelectronic systems such as phased-array antennas and antenna remoting thus often combine radio-frequency mixing operations and electro-optic modulating operations. Particularly in a phased-array antenna, these optoelectronic systems typically involve excessively large numbers of radio-frequency mixers (and attendant amplifiers) and electro-optic modulators.
In addition, the electro-optic modulators have a number of problems. Their modulation is sensitive to the polarization of optical signals so they must be connected in a system with polarization-maintaining optical fibers. These fibers are quite expensive and their use in long distance applications, e.g., antenna remoting, adds considerable system cost. In addition, the modulators have an optical conversion loss (typically &gt;3 dB) and their performance is sensitive to temperature.
Accordingly, development efforts have been directed to mixing and modulating processes and structures which can reduce these large parts counts in optoelectronic systems by combining radio-frequency mixing and optical modulating operations. Preferably, such processes and structures would produce less optical conversion loss than electro-optic modulators and eliminate the need for expensive polarization-maintaining optical fibers.
An exemplary development is a millimeter-wave photonic downconverter which was described by Logan, Ronald T., et al. (Logan, Ronald T., et al., "Millimeter-Wave Photonic Downconverters: Theory and Demonstration", Proceedings of SPIE Conference on Optical Technology for Microwave Applications VII, Jul. 9-14, 1995). The downconverter includes a Mach-Zehnder modulator and a dual-frequency laser. The laser generates a dual-frequency optical signal having a frequency separation between the optical modes equal to a desired LO frequency.
This optical LO signal is applied to the optical input port of the Mach-Zehnder modulator and an RF signal is applied to the modulation port of the modulator. For example, Logan, et al. coupled a 28 GHz RF signal and a dual-frequency laser with a frequency separation of 8.6 GHz to a Mach-Zehnder modulator to generate a 19.4 GHz difference signal and a 36.6 GHz sum signal which were then recovered from the optical carrier signal with a photodetector. Although this structure generates sum and difference signals in an optical carrier without the need for radio-frequency mixers, it relies upon an electro-optic modulator which has the previously mentioned problems.